Text Panel: add variables representing current hostname, port, base path to allow more flexible link creation

<!-- Please only use this template for submitting feature requests --> I have a Grafana instance with Text Panel called *Welcome* describing what dashboards are available, how to use them etc. I'd like to include links to various other related services running on the same machine as Grafana (such as Prometheus Datasource, NGINX proxy, related web app) that are being monitored, etc. (See the screenshot below). It is currently possible to link to such services by hardcoding the link either in Markdown or in the HTML. for example by referencing: - `https://mygrafana.example.com:8080/another-app` - `/d/dashboards/...` This works when you know the machine name, and when you know where precisely is Grafana running. But If I have no permanent hostname such as for example a Grafana running only on an IP address, or if I have it served via subpath that I may change, I have no way to form such links at all. Neither in Markdown, nor in HTML, since HTML is sanitised and can not use Javascript to infer `window.location`. <img width="1517" alt="image" src="https://github.com/grafana/grafana/assets/249814/5fca3088-7705-42cd-b622-db0a0727d3d6"> Text panel currently provides several variables: - `$__from` - `$__to` - `$__url_time_range` **What would you like to be added**: I propose to add the following variables to essentially mimic the Javascript `window.location` object. - `$__grafana_proto`: for example `http` or `https` - `$__grafana_host`: for example `mygrafana.example.com` or `192.168.1.10` - `$__grafana_port`: for example `3000` - `$__grafana_base_url`: for example `/apps/grafana` in case Grafana is served from subpath instead of `/`. to represent where the Grafana is served from and to allow semi-dynamic creation of links. **Who is this feature for?** Anybody working with Text Panels and wishing to link out to related services/apps running on the same machine as Grafana without necessarily knowing how the Grafana will be deployed.